The cells of plants and most of the epidermal (skin) cells of animals are capable of sensing mechanical touch, to which they respond by producing a complex electrical signal. When an improved capability for sensing mechanical touch is essential for the functioning of an organ, the sensitivity may be enhanced by surface extrusions in the form of either sensory hairs, such as are found on the upper side of the leaf pair of the Venus flytrap and the lower part of leaf joints of Mimosa pudica, or the sensory papillae that are found on the surface of tendrils in some species of Cucurbita (a plant family commonly known as melons, gourds or cucurbits) and Passiflora (a plant family commonly referred to as passion vines). Instead of distributing the enhanced sensitivity over many cells, these structures focus the response to a touching force so that it occurs on the surface area of only one cell—the sensor cell—and thus, amplify the resulting hydraulic pressure developed by an applied force.
A close look at sensory papillae (small projections on leaf or vine surfaces) of such plants (for example, papillae 24 that are shown on a trichome 22 in a scanning electron micrograph 20 in FIG. 1), reveals that this mechanosensor is not just a pressure sensor capable of sensing pressure (or force) applied only along the direction perpendicular to the surface of the plant tissue (i.e., in the direction designated by a vector Fz), but also has a capability of sensing forces applied in other directions, i.e., forces that are incident on the surface of the generally dome-shaped papillae at small acute or obtuse angles (see, for example, a model 26 of a plant tactile sensing papilla dome structure 28 shown in FIG. 2). Therefore, these mechano or tactile sensors in certain plants can sense the surface topology of a foreign object that touches the sensors. This tactile sensitivity can help plant tendrils, such as on cucumber vines, search for stable objects with rough surfaces so that the tips of the tendrils can grab the objects that are contacted, coiling around the objects for support, enabling the plant vines to spread more efficiently.
The mechanisms inherent in the sensory papillae of the Cucurbitaceae plant family appear to be driven by both hydrated ion motion and stretch/contraction and/or re-orientation of fibrils embedded in the sensor cells. The former mechanism is referred to as “hydrated ion motion,” and the latter as “tensegrity network motion,” where a number of fibrils form a connected network. Thus, any mechanical stimulus at one location can transduce a signal through the network of the fibril microstructure. Accordingly, it might be desirable to employ an analogue of the hydrated ion motion mechanism inherent in the sensor cells of plants, in the design of man-made tactile sensors.
Man-Made Tactile Sensors
Most man-made tactile sensors can detect pressure from only one direction. For example, a typical tactile sensor may include a cantilever that responds to a force that deflects the distal end of the cantilever. In contrast, the tactile sensors in certain plants can detect “vector” forces, i.e., can react to the direction of an applied force that may be applied from any of a number of different directions. A comparison between the natural hair sensors in the plant known as the Venus Fly Trap, and the sensor cells in Cucurbitaceae indicates that the former can sense only forces directed along one of two orthogonal directions, Fx or Fy, while the latter senses the components of forces directed along a plurality of orthogonal directions, i.e., Fx, Fy, and Fz. In this sense, the natural tactile sensor of the papillae in Cucurbitaceae possesses a well-developed sensor microstructure, and its distributed pattern on the surfaces of vines and leaves comprises a good design for an arrayed tactile sensor system useful for identifying the orthogonal components of applied forces so as to determine the direction of the force.
Most of the man-made tactile sensors that have been developed so far are based on piezoelectric sensing elements covered by an elastomeric top coating, or on piezo-resistive elastomers. Both types of sensors are useful for measuring pressure distribution in a robotic hand, where constant electric power can be applied to the sensors during the time they are employed for sensing force. However, such sensors will require either a fixed source of power or a portable battery supply, which can limit their usefulness. It would be preferable if new types of tactile sensors might be developed that do not require any power supply, yet can provide a full set of sensing signals in response to applied mechanical forces.
Other currently available commercial sensors are based on silicon-micro-electrical mechanical systems (MEMS) technology that exhibit a touch sensitivity that is not very linear, or on conducting polymers made of conductive fillers and elastomers having a touch sensitivity that is more linear than that of Si-MEMS, but relatively low in sensitivity.
The response to force by a tactile sensor should not be limited to only a force applied in the direction, but instead, the sensor should also respond to force components in all of the orthogonal directions, i.e., to force components Fx, Fy, and Fz. Again, it would be desirable to generally duplicate in a man-made tactile sensor, the ability to sense the direction of an applied force, like the papillae sensors in certain plants.
Materials that may be useful in providing such tactile sensors include electroactive polymers (EAP) made with ion-exchange membrane materials such as Nafion™ and Flemion™. Nafion™, which was originally developed for use in reverse osmosis desalination by DuPont, was first used as an actuator material by the Oguro group at the Osaka National Research Institute, Japan, in 1992, which determined that it deforms in response to an applied voltage. Following the pioneering work by Oguro et al., a number of other researchers have continued to study the uses of EAPs. For example, mechanical-actuators based on EAPs have also been developed using Flemion™, and polyvinyl alcohol (PVA). Like Nafion™, Flemion™ was originally developed as an ion-exchange membrane, with application to fuel cell technology, but was subsequently shown to be useful in actuators.
Recently, a charge sensing model has been developed that is based on proportionality between stress and charge density. This model predicts that an induced stress will produce a capacitive discharge in a polymer, such as an EAP, and the prediction has been verified through a series of experiments. By monitoring the voltage of the discharge that is produced, the EAP can produce an indication of applied force and can be used as a tactile sensor. This work was initially carried out using an EAP constructed with Nafion™. However, the relatively low durability of a tactile sensor based on Nafion™ would limit it to a very short useful life. Therefore, it is clear that more durable EAP-based tactile sensors would be favored, because of their sensitivity, flexibility, ease of fabrication, and low cost. In addition, because a source of electrical power is not required to energize EAP tactile sensors during the sensing process, an EAP-based device would have minimal energy consumption, making it very useful in applications in which the sensor cannot be continuously connected to a power supply while needed for tactile sensing.